Heat Capacity and Enthalpy Behavior Near Phase Transitions in Some Alkylcyanobiphenyls

Abstract

An adiabatic scanning calorimeter has been used in a calorimetric investigation of alkyl-cyanobiphenyl (nCB) liquid crystals. Measurements near solid to smectic (KA), smectic A to nematic (AN) and nematic to isotropic (NI) or smectic A to isotropic (AI) phase transitions have been carried out for scanning rates as low as a few μKs⁻¹. Results for the temperature dependence of the heat capacity as well as for the enthalpy (including latent heats) have been obtained for 8CB, 9CB, 10CB and 11CB. Latent heat values are reported for the different KA, NI and AI transitions. For the two compounds 8CB and 9CB, which show a AN transition, it was found that, within the resolution of the experiments, these transitions are second-order. The anomalous heat capacity observed near the AN transition is much larger for 9CB than for 8CB. The critical exponent α obtained from power law fits to the heat capacity data is 0.31 ± 0.03 for 8CB and 0.50 ± 0.05 for 9CB. It is suggested that the AN transition in 9CB must be very close to a tricritical point, and that the α value for 8CB should be considered as an effective value, reflecting crossover between the tricritical and XY critical behavior.